Chromate-Free Corrosion Inhibitors Remain Beyond Reach

Aerospace, by necessity, is a forward-thinking industry. Eleven decades ago, there was no such thing as an airplane. Today's trade headlines are sprinkled with references to advances like pyrolysis oil-based fuels and carbon fiber reinforced plastic airframes.

Sometimes, however, ensuring tomorrow's success requires hanging on to a bit of yesterday.

That's the position that several aviation organizations, including the Association of European Airlines (AEA), have taken regarding REACH, the European Community's wide-ranging regulation that, among other things, requires chemical manufacturers to developed substitutes for certain substances deemed to be dangerous to people or the environment.

Enacted in 2007, REACH (short for Registration, Evaluation, Authorization and Restriction of Chemicals) “is based on the idea that industry itself is best placed to ensure that the chemicals it manufactures and puts on the market in the EU do not adversely affect human health or the environment,” the EU explains in a background document on the program. “This requires that industry has certain knowledge of the properties of its substances and manages potential risks.”

The program includes a list of Substances of Very High Concern (SVHC). Applicants who want to continue using SVHC “will have to demonstrate that risks associated with uses of these substances are adequately controlled or that the socio-economic benefits of their use outweigh the risks,” the EU explains. Applicants also must determine whether a safer alternative exists. If SVHC items are deemed harmful enough, they are given “sunset dates” beyond which they are forbidden for use in the European Union, unless a special exception is given. Exceptions are temporary, however, and are only granted if the applicant also provides a permanent substitution plan for the SVHC.

The SVHC list started with 15 substances in 2008 and has grown to 53. Thirteen of them are chromates, such as potassium chromate, which inhibits corrosion on aluminum, and—as AEA notes in a position paper published in October 2011—is lauded for its effectiveness in preventing further degradation in places that already have been damaged. Aerospace companies use other chromates in metal processing, such as chrome plating, conversion coatings, black oxide coatings, deoxidizers and cleaning titanium, AEA says.

AEA suggests that any chromate used in the following processes should be exempted: corrosion inhibitors, metal finishing, sealants, chemical stripping and specialty coatings.

“These specialty uses have no off-the-shelf alternatives available today,” AEA says. “Corrosion protection being an essential defense mechanism for metallic components, many of the applications can be considered to be safety critical applications. “ As such, AEA reasons, they should be exempt from the SVHC process.

AEA argues that replacing chromates with less-than-equivalent substances could create unsafe conditions, such as stress corrosion cracking, corrosion fatigue, exfoliation and other types of corrosion. “Pitting corrosion can also lead to fatigue failures, and general corrosion may extend to the point that the metal loss affects structural properties,” AEA notes. “Given the complex geometry of aerospace construction, such corrosion may not be apparent through routine inspection and maintenance before reaching a failure point. This represents an important safety risk for users.”

Another challenge, AEA points out, is the long lifecycle of a typical aircraft. “[A]lternatives must be compatible with existing aircraft support systems,” AEA says. “Forced substitution would be incompatible with spare and maintenance aftermarkets.”

The aerospace industry has been researching potential alternatives for several years, AEA notes. Unfortunately, equivalent substances remain elusive. “Although significant research efforts are still ongoing, no drop-in alternatives exist today or should be expected for a majority of aerospace uses in the near future.”

AEA's white paper was presented after the public comment period for adding substances to the SHVC list, although the European Chemicals Agency (ECA), the body charged with implementing REACH, notes that the aerospace industry submitted detailed comments during the period. ECA also says that while exemptions would be considered for individual substances, the REACH process does not allow for entire industry exemptions.